**6.2 Gas processing unit**

Chandrasekar (2006) demonstrated the gas processing unit (GPU). In stage one biogas from the digester will be cleaned of moisture droplets, particulates and hydrogen sulfide. The cleaned gas mixture, which consists primarily of methane (CH4) and carbon dioxide (CO2), will be then converted in stage 2 to ultra-high purity hydrogen in a steam reformer. As a first step to realize this vision, a GPU was installed (Fig. 19) which has been successfully removing over 99% of hydrogen sulfide (H2S) along with most of the water droplets and particulates. A steam reformer has been also installed.

Fig. 19. Activated carbon beds of the GPU (Chandrasekar, 2006)

In the GPU biogas from the digester is pressurized to over 3 inches water column by a blower. It then passes through a coalescing filter to remove most of the particulates and water droplets. Water collected in the coalescing filter gets automatically drained out once it reaches a certain level. The biogas is then heated to about 85 oF in a heater before it passes through two successive activated carbon beds where H2S is converted into elemental sulfur. The process has been optimized so that bed replacement is needed only once every six months. The configuration of dual beds allows for continuous operation even when one bed is being replaced. The bed manufacturer should be contracted to replace the used beds, thereby obviating the need for the farmer to handle the sulfur. The design requires minimum operation and maintenance and has been set up to be controlled through a computer that will also monitor the incoming gas pressure, control and monitor the blower as well as monitor the exit H2S concentration and shut the blower/GPU if the exit concentration is greater than the set point. If the GPU shuts down, biogas will automatically feed the engine generator like before to produce electricity. A simple schematic of the GPU is shown in Figure 20.

Chandrasekar (2006) demonstrated the gas processing unit (GPU). In stage one biogas from the digester will be cleaned of moisture droplets, particulates and hydrogen sulfide. The cleaned gas mixture, which consists primarily of methane (CH4) and carbon dioxide (CO2), will be then converted in stage 2 to ultra-high purity hydrogen in a steam reformer. As a first step to realize this vision, a GPU was installed (Fig. 19) which has been successfully removing over 99% of hydrogen sulfide (H2S) along with most of the water droplets and

**6.2 Gas processing unit** 

is shown in Figure 20.

particulates. A steam reformer has been also installed.

Fig. 19. Activated carbon beds of the GPU (Chandrasekar, 2006)

In the GPU biogas from the digester is pressurized to over 3 inches water column by a blower. It then passes through a coalescing filter to remove most of the particulates and water droplets. Water collected in the coalescing filter gets automatically drained out once it reaches a certain level. The biogas is then heated to about 85 oF in a heater before it passes through two successive activated carbon beds where H2S is converted into elemental sulfur. The process has been optimized so that bed replacement is needed only once every six months. The configuration of dual beds allows for continuous operation even when one bed is being replaced. The bed manufacturer should be contracted to replace the used beds, thereby obviating the need for the farmer to handle the sulfur. The design requires minimum operation and maintenance and has been set up to be controlled through a computer that will also monitor the incoming gas pressure, control and monitor the blower as well as monitor the exit H2S concentration and shut the blower/GPU if the exit concentration is greater than the set point. If the GPU shuts down, biogas will automatically feed the engine generator like before to produce electricity. A simple schematic of the GPU

Fig. 20. Schematic of the GPU (Chandrasekar, 2006)

#### **6.3 Mixing technology**

The types of mechanical mixing (Fig. 21) are: vertical mixing, horizontal mixing, and side mixing. Submersible motor mixing devices are usually used in commercial biogas plants. Each device is provided by a cable and gear protection system (Fig. 22). Light agitation increases the velocity of digestion, differently from heavy agitation which decreases the velocity of reaction. In digesters with capacities higher than 100 m3, it is necessary to install equipment to provide agitation of the contents.

Fig. 21. Mechanical mixing

Biogas Plant Constructions 363

The solids feeder (Fig. 23) is a device that feeds the digester by the solid biowastes, i.e. dry organic matter. The solids feeder consists of a dosing container, weighing cells, identification/weighing system, and a mechanical system. The mechanical system of the solids feeder consists of a hydraulic drive system for the pushing floor, cross feeder auger drive, transport auger drive, transport auger, tamping auger drive, and tamping auger.

(a)

(b)

Fig. 23. Solids feeder (MT-ENERGIE GmbH & Co. KG)

**6.4 Solids feeder** 

(c) Cable implementation to mixer

Fig. 22. Gear protection system (MT-ENERGIE GmbH & Co. KG)
